The immune system functions as the body's major defense against diseases caused by invading organisms. This complex system fights disease by killing invaders such as bacteria, viruses, parasites or cancerous cells while leaving the body's normal tissues unharmed. The immune system's ability to distinguish the body's normal tissues, or self, from foreign or cancerous tissue, or non-self, is an essential feature of normal immune system function. A second essential feature is memory, the ability to remember a particular foreign invader and to mount an enhanced defensive response when the previously encountered invader returns. The loss of recognition of a particular tissue as self and the subsequent immune response directed against that tissue produce serious illness.
An autoimmune disease results from the immune system attacking the body's own organs or tissues, producing a clinical condition associated with the destruction of that tissue. An autoimmune attack directed against the joint lining tissue results in rheumatoid arthritis; an attack against the conducting fibers of the nervous system results in multiple sclerosis. The autoimmune diseases most likely share a common pathogenesis and the need for safe and effective therapy.
Rheumatoid arthritis is one of the most common of the autoimmune diseases. Current treatments utilize three general classes of drugs (Schumacher, 1988): antiinflammatory agents (aspirin, non-steroidal anti-inflammatory drugs and low dose corticosteroids); disease-modifying antirheumatic drugs, known as "DMARDs" (antimalarials, gold salts, penicillamine, and sulfasalazine) and immunosuppressive agents (azathioprine, chlorambucil, high dose corticosteroids, cyclophosphamide, methotrexate, nitrogen mustard, 6-mercaptopurine, vincristine, hydroxyurea, and cyclosporin A). None of the available drugs are completely effective, and most are limited by severe toxicity.
In addition to their use in treating autoimmune conditions, immunosuppressive agents have also been used in treating or preventing transplantation rejection. Organ transplantation involving human organ donors and human recipients (allografts), and non-human primate donors and human recipients (xenografts), has received considerable medical and scientific attention (Roberts, 1989; Platt, 1990; Keown, 1991; Wang and Morris, 1991; Hasan, 1992; Murase, 1993). To a great extent, these efforts has been aimed at eliminating, or at least reducing, the problem of rejection of the transplanted organ. In the absence of adequate immunosuppressive therapy, the transplanted organ is destroyed by the host immune system.
Another obstacle in transplantation, which has limited bone marrow transplants (BMT) in particular, is graft-versus-host disease (GVHD). GVHD is a condition in which transplanted marrow cells attack the recipient's cells (Thomas, 1975; Storb, 1984). Many BMT patients receiving HLA-identical marrow that tests negative in the mixed lymphocyte reaction (MLR) still develop GVHD, presumably because of a disparity between the recipient and donor at polymorphic non-HLA determinants. A large proportion of GVHD-afflicted individuals die as a result of GVHD (Weiden, et al., 1980).
Presently, the most commonly used agents for preventing transplant rejection include corticosteroids, antimetabolite drugs that reduce lymphocyte proliferation by inhibiting DNA and RNA synthesis such as azathioprine, immunosuppressive drugs such as cyclosporin A, which specifically inhibits T cell activation, and specific antibodies directed against T lymphocytes or surface receptors that mediate their activation (Briggs, 1991; Kennedy, 1983; Storb, 1985; Storb, et al., 1986). All of these drug therapies are limited in effectiveness, in part because the doses needed for effective treatment of transplant rejection may increase the patient's susceptibility to infection by a variety of opportunistic invaders, and in part because of direct toxicity and other side effects. For example, cyclosporin A, currently the most commonly used agent, is significantly toxic to the kidney. This nephrotoxicity limits the quantity of drug that can be safely given.
Recently, a number of compounds from the Chinese medicinal plant Tripterygium Wilfordii (TW) have been identified as having immunosuppressive activity. Representative compounds which have been isolated from TW include triptolide, 16-hydroxytriptolide, triptophenolide, tripdiolide, and celastrol, as described for example in Lipsky, et al. (1994) and Zheng et al. (1991; 1994). However, the administration and therapeutic effectiveness of these compounds have been limited by their low water solubility.
One approach for improving the effectiveness of these compounds is to formulate them in mixtures of ethanol and polyethoxylated castor oil (e.g., "CREMOPHOR EL"), allowing subsequent dilution in saline for intravenous administration. However, such formulations have suffered from high toxicity, due to the high concentration of solubilizing agent required to dissolve these compounds. For example, the ratio of solubilizing agent (ethanol plus "CREMOPHOR EL") to triptolide in such formulations is typically on the order of 1000:1 or greater, due to the poor solubility of triptolide (Morris, 1991; Morris, et al., 1991). Standardization of dosage amounts is also more problematic with a suspension than with a solution.
It would therefore be desirable to provide immunosuppressive compounds having improved water solubility and low toxicity. In addition, it would be desirable for such compounds to exhibit immunosuppressive activity in their water soluble form, or to be convertible to an immunosuppressive form by metabolic processes in vivo.